1. Field of the Invention
The invention relates generally to systems and methods for distributing accurate time and frequency for networked receivers and, in particular, to systems for distributing time and frequency through the use of radiated “signals of opportunity,” such as local radio or television signals.
2. Background Information
Timing is critical for networks, for example, in high speed wireless networks in which handoffs occur. Another example of network in which timing is critical is a city power distribution network in which high voltage direct current (DC) transmission lines are used to transfer power over long distances between power generation facilities, such as hydroelectric dams, and consumers. The high voltage DC power signals must be converted to low voltage AC power signals before distribution to businesses and households. Cities generally have a number of power transformer stations that are interconnected on power grids to do the conversion. The interconnected power transformer stations must be producing the low voltage AC power signals in exactly the same frequency and phase before the signals can be distributed across common low voltage transmission lines. Accordingly, the power transformer station operations must be synchronized in time and frequency. There is thus a need for accurate time determination as well as accurate frequency determination at distributed, or remote, locations within the network.
GNSS receivers at the remote locations can provide timing and frequency information, as long as the antennas at the remote locations have sufficiently clear views of the sky. However, remote locations that are within cities often do not have such views of the sky, and there is thus a need for providing timing and frequency information at the remote locations.